In a conventional electrostatographic copying process, a latent electrostatic image is formed on the insulating surface of a photoconductor element. If a dry development process is used, charged toner particles are applied to the electrostatic image, where they adhere in proportion to the electrostatic potential difference between the toner particles and the charges on the latent image. Toner particles that form the developed image are then transferred to a receiver sheet, where the transferred image is fixed, usually by a thermal fusion process in which the receiver sheet is passed between a pair of rollers under pressure and subjected to temperatures of about 200-300.degree. F. (93-149.degree. C.). It is conventional to transfer toner particles from the photoconductor element to the image receiver sheet by means of an electrostatic bias between the element and the receiver sheet.
While the conventional electrostatic transfer process works well with large toner particles, difficulties arise as the size of the toner particles is reduced. Smaller toner particles are necessary for images of high resolution and low granularity. As the particle size of the toner falls below about 8.mu., however, the surface forces holding the toner particles to the element tend to dominate over the electrostatic force that can be applied to the particles to assist their transfer to the receiver sheet. Thus, less toner transfers, and image quality suffers. In addition, as the particle size decreases, coulombic repulsion between the particles tends to scatter them, causing loss in image resolution and increase in graininess and mottle. Thus, high resolution images require very small particles, but it is difficult to obtain high resolution electrostatic transfer images without image defects.
To aid in transferring all of the toner particles from the element to the receiver, it is advantageous to coat the image-receiving surface of the sheet with a thermoplastic polymer. During transfer, the toner particles adhere to or become partially embedded in the thermoplastic coating and are thereby more completely removed from the photoconductor element. A further improvement in toner transfer may be obtained by coating the thermoplastic polymer layer on the receiver sheet with a release agent. However, if the binder resin for the photoconductor and the thermoplastic polymer layer of the receiver sheet are appropriately selected with respect to their compositions and surface energies, a release agent is not necessary.
Receiver sheets for electrophotographic toner images are most often paper, although plastic sheets have also been used. Both have disadvantages, especially for receiving fusible toner powder of small particle size in the making of continuous tone or half-tone electrophotographic reflection prints. To use a conventional transparent plastic sheet for this purpose, the plastic must be pigmented with, for example, titanium dioxide or the like in order to provide an opaque reflective support for the toner image. Blending a colorant with the polymer adds cost, and the pigmented sheet has a higher specific gravity. Furthermore, colorants tend to fade or otherwise change color with aging.
As for paper, its untreated surface is typically too rough to give high resolution transfer images. Consequently, a smooth surface must be produced, either by calendering or by applying a layer of plastic or clay to the paper, which adds cost. A particularly serious disadvantage of a paper receiver sheet is that, being fibrous and hydrophilic, it unavoidably contains moisture. When heated, as in the toner fusing step, the moisture in the paper vaporizes and causes buckling and blistering in the toned image, especially in large areas of toner. Furthermore, a paper receiver sheet upon exposure to water is prone to distortion, tearing, and other damage.
U.S. Pat. No. 4,795,676, the disclosure of which is incorporated herein by reference, describes an electrostatic recording material composed of a multi-layered synthetic paper support having an electroconductive layer and a dielectric layer formed successively thereon. The support has a base layer, with paper-like layers of thermoplastic resin on both sides, and surface layers of thermoplastic resin containing little if any inorganic fine powder.
U.S. Pat. No. 5,055,371, the disclosure of which is incorporated herein by reference, describes a receiver sheet for toner images that comprises a paper-like, substantially opaque microvoided polymeric sheet of a continuous matrix of oriented and heat set thermoplastic polymer in which is dispersed polymeric microbeads surrounded by void spaces. Bonded to at least one surface of the microvoided polymeric sheet is a layer of thermoplastic polymer whose glass transition temperature is below the melting temperature of the matrix polymer of the microvoided sheet.
JP 1197763 discloses a paper with more than 65 percent opacity that is suitable for use with a non-impact printer. The paper is characterized as having a coating agent composed of 80-40 weight percent of an acrylic urethane resin and 20-60 weight percent of a filler on the surface of a synthetic paper whose surface layer is a stretched polyolefin film that contains 20-65 weight percent of an inorganic fine powder.
JP 3234588 discloses an image receiving sheet for a thermal transfer printer that includes a base of monoaxial or biaxial drawn polyolefin film that has a resin-coated layer on its front and rear surfaces, with a color image receiving layer composed mainly of saturated polyester and crosslinking agent.
JP 6324509 discloses a toner receiving sheet for color electophotography containing a resin with a T.sub.g of -20.degree. C. to +30.degree. C. and spherical low molecular weight polyolefin with a softening point of 100.degree. C. or more, the average particle diameter being 0.1-1.0 .mu.m.
JP 1006958 discloses a static recording sheet comprising a substrate, preferably a synthetic paper, with an electroconductive layer and a dielectric layer disposed on at least one surface of the substrate.
JP 5169864 discloses an image receiving sheet for thermal printing comprising: a surface layer of a single-layered drawn porous film that has a void structure and contains a thermoplastic resin and an inorganic pigment as the main components; a back layer that also contains a thermoplastic resin and an inorganic pigment but is a multi-layered porous film; and an image receiving layer.
JP 4039089 discloses an image receiving paper for sublimation heat transfer having an intermediate layer formed of an addition-polymerizable composition and a polyolefin resin and an outer layer of a thermoplastic polyester resin provided in order on a highly smooth base material.
A need exists for an improved receiver sheet, especially for images containing large solid areas of toner. Such a sheet must meet several important criteria. First, it must be suitable for the fusion and fixing thereto of toner powders of small particle size to provide images of high resolution. The sheet must retain dimensional stability when heated during the transfer and fixing of toner to it. The sheet must be highly moisture-resistant to avoid problems caused by water vaporization during heating, and also to provide protection, if exposed to water, to tearing or other damage. In addition, there must be good adhesion between the thermoplastic surface layer and the substrate of the receiver sheet to avoid delamination when heated. Then, of course, as a support for electrophotographic prints, the sheet must be substantially opaque and highly reflective for visible light. For convenience in handling, the sheet should be flexible and of reasonably low specific gravity.
The present invention offers further improvement in the forming of images of high resolution, especially when the toner images have large solid areas of toner. Images of this kind include, in particular, continuous tone electrophotographic color prints, but also half-tone images in which dot spread occurs to create large solid toner areas, as well as largely alpha-numeric images that include solid areas such as graphics and corporate logos.
A problem with all such images, when paper is the receiver sheet substrate, is that the toner in the large solid areas will crack as a result of deformation of the paper caused by water absorption. When the paper dries out, it shrinks unevenly, relatively less in large toner areas. Variable dimensional changes across the receiver sheet surface would damage any continuous tone or half tone images having large toner areas. The paper may also curl or wrinkle. Ordinary plastic sheets, although not moisture-absorbent, also have drawbacks, as mentioned previously. All these problems are overcome by the toner image receiver sheet of the present invention.